Is BMI the Best Tool to Determine Healthy Weight? The WHO today recognizes the validity of the Body mass index (BMI) and has set the following ranges to determine the distinction between being thin and being overweight or obese. Classification BMI Range Normal 18.5 and 24.9 Underweight Under 18.5 Overweight 25 to 29.9 Obese 30.39.9 Calculate your BMI To calculate your BMI, simply divide your weight—measured in kilograms—by your height squared—measured in meters Limitations Height The mathematics of calculating BMI by dividing weight means that it returns too large a number for short people and too small a number for ta